This application claims the priority of Japanese Patent Application No. 2001-168242 filed on Jun. 4, 2001, which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a support apparatus for an optical wave interferometer reference plate, which is used for allowing an optical wave interferometer body to support a reference plate employed in an optical wave interferometer capable of measuring the surface form or internal distortion of optical components and the like with a high accuracy.
2. Description of the Prior Art
Various optical wave interferometers have been in use in order to measure very fine surface forms of samples and the like accurately, and so forth. In optical wave interferometers carrying out such highly accurate measurement, the reference plate is required to be prepared with a high precision as well.
Therefore, though depending on the accuracy required in measurement, the reference surface of a reference plate has been finished grinding with such a high precision that its amount of deviation from a perfect plane is {fraction (1/20)} of the optical wavelength in use or so in general. Though the rear side of the reference surface is not required to have accuracy as high as that of the reference surface, it has also been finished grinding with a precision of about ⅕ to {fraction (1/10)} of the optical wavelength in use.
The outer peripheral face of the reference plate has been ground or lapped with a roughness of about #400 to #800 in general. Also, the reference plate has been formed from a material having a low coefficient of thermal expansion, such as silica or ceramics in general. Therefore, in optical systems in which luminous fluxes are transmitted through a reference plate in particular, evenness is required in the refractive index distribution within the material.
It is important that a reference plate finished with a high precision as such be attached to an interferometer body and used while keeping its precision. Therefore, various methods for supporting a reference plate have been proposed and used conventionally.
Some of typical methods have been disclosed in commonly assigned Japanese Unexamined Patent Publication No. 2000-249512.
For example, there is a method in which a reference plate is supported from its optical axis direction end face (reference surface) side. This method is one in which a lens barrel for supporting a reference plate is formed with an annular abutment surface for supporting the reference surface of the reference plate, so that the reference plate is supported with the abutment surface from the reference surface side. However, since the machined abutment surface has a surface precision inferior to the reference surface finished grinding with a high precision in this method, the contact between the abutment surface and the reference surface may become uneven, so that the reference plate may flex unevenly, whereby the surface precision of reference surface may deteriorate.
Hence, a method in which a cushioning material is held between the reference surface and the annular abutment surface has been proposed in order to alleviate the uneven contact. However, though the uneven contact is alleviated by this method, the cushioning material may lose its elasticity with time due to the continuous pressure effected by the weight of the reference plate itself, so that the cushioning effect may be lost, whereby uneven contact may occur again.
Therefore, a method in which the reference plate is supported from its diametric end face (outer peripheral face) side has been proposed. This method bonds the outer peripheral face of the reference plate to the inner face of the lens barrel such that the reference plate is suspended with respect to the lens barrel, whereas an adhesive is mainly used for bonding. Since nothing comes into contact with the reference surface of the reference plate in this method, the problem of surface precision deterioration resulting from the contact between the reference surface and the abutment surface or cushioning material as in the method supporting the reference surface of the reference plate does not occur.
However, even in this supporting method, the material forming the lens barrel and the material forming the reference plate may have coefficients of thermal expansion different from each other, so that a force may act on the reference plate by way of the bonded part of the outer peripheral face of the reference plate when temperature changes, thereby deforming the reference surface having a high precision.
Also, using an adhesive for bonding may be problematic when curing the adhesive. Namely, if the thickness of an adhesive layer varies, a force may act on the reference plate due to volumetric changes upon curing, thereby altering the reference surface having a high precision.
Therefore, a method in which the reference plate is elastically supported from its diametric end face (outer peripheral face) side has been devised. In this method, an elastic member is disposed at the outer peripheral face of the reference plate, and the lens barrel and the outer peripheral face of the reference plate are secured to each other by way of the elastic member such that the reference plate is suspended with respect to the lens barrel. Since nothing comes into contact with the reference surface of the reference plate in this method, the problem of surface precision deterioration resulting from the contact between the reference surface and the abutment surface or cushioning material as in the method supporting the reference surface of the reference plate does not occur. Also, since the reference plate and the lens barrel are not in contact with each other directly but by way of the elastic member, no force acts on the reference plate by way of the bonded part of the outer peripheral face of the reference plate when temperature changes, and no force acts on the reference plate due to volumetric changes upon curing the adhesive, whereby the reference surface having a high precision does not deform.
However, since the elastic member is disposed at the outer peripheral face of the reference plate, whereby the lens barrel and the outer peripheral face of the reference plate are secured to each other by way of the elastic member such that the reference plate is suspended with respect to the lens barrel, the above-mentioned supporting method may also be problematic in that the reference surface position varies along with temporal changes in elastic force of the elastic member.
Also, when the reference plate is measured while repeatedly moving/stopping it in its optical axis direction at a high speed in order to obtain highly accurate numeric data by interference measurement, as in the case using fringe scanning, which is a highly accurate interference fringe analyzing technique, minute vibrations corresponding to the spring constant of the elastic member for holding the reference plate with respect to the lens barrel in a suspended state may occur at the time when the reference plate starts and stops moving, thus making it difficult to carry out highly accurate measurement.
Further, when the lens barrel having the reference plate suspended by way of the elastic member is attached to the interferometer body, the reference plate and the interferometer body are elastically secured to each other. Since the characteristic frequency of the interferometer body and that of the reference plate differ from each other, the reference surface and the surface of a sample to be inspected may not stand still relative to each other depending on the vibrating environment in which an interferometer apparatus having the reference plate attached thereto is placed.
In view of the circumstances mentioned above, it is an object of the present invention to provide a support apparatus for an optical wave interferometer reference plate which, when supporting the outer peripheral face of the reference plate, can prevent forces generating deformations influential in the accuracy in measurement from acting on the reference plate due to the difference in coefficient of thermal expansion between the support apparatus and reference plate when temperature changes and prevent the reference surface from moving or vibrating relative to the surface of a sample to be inspected.
In particular, it is an object of the present invention to provide a support apparatus for an optical wave interferometer reference plate which, when bonding the reference plate to the support apparatus in a suspended state, can prevent forces generating deformations influential in the accuracy in measurement from acting on the reference plate due to the difference in coefficient of thermal expansion between the support apparatus and reference plate when temperature changes, restrain forces from acting on the reference plate due to volumetric changes upon curing an adhesive, and prevent the reference surface from moving or vibrating relative to the surface of a sample to be inspected.
For achieving the above-mentioned objects, the support apparatus for an optical wave interferometer reference plate in accordance with the present invention is configured such that the reference plate is attached to a lens barrel or an interferometer body by way of a support member bonded to the outer peripheral face of the reference plate, the support member can be constructed by a single component, and the difference in deformation between the support member and reference plate occurring due to the difference in their coefficients of thermal expansion at the time when temperature changes can be cancelled by diametric/circumferential elastic deformations of the support member.
Namely, in an optical wave interferometer for splitting a luminous flux from a light source into two, irradiating a sample with one of thus obtained two luminous fluxes so as to attain object light carrying a phase state of the sample, irradiating a reference plate with the other so as to attain reference light carrying a phase state of the reference plate, and re-combining the object light and reference light together so as to attain an interference fringe corresponding to a phase difference therebetween, the support apparatus for an optical wave interferometer reference plate in accordance with the present invention comprises a support member for supporting an outer peripheral face of the reference plate, the support member being a structure bonded to the outer peripheral face of the reference plate at a plurality of positions spaced from each other along the circumferential direction of the outer peripheral face and adapted to deform elastically in a circumferential/diametric direction of the reference plate but less in an optical axis direction of the reference plate than in the circumferential/diametric direction.
The support member may have an annular form surrounding the outer peripheral face of the reference plate, while comprising cutouts extending from one of first and second ends in the optical axis direction of the reference plate toward the other end to a position near the other end, in which cutouts extending from the first end toward the second end and cutouts extending from the second end toward the first end substantially alternate with each other along the circumferential direction of the support member.
The support member may be formed with adhesive injection holes penetrating through the support member from the outer peripheral face to inner peripheral face thereof with a predetermined interval along the circumferential direction of the support member, whereas a groove extending in the circumferential direction may be formed at positions where the adhesive injection holes are formed.
The support member may have an annular base disposed at a position separated from the outer peripheral face of the reference plate in the optical axis direction of the reference plate, a plurality of support arms extending from the base in the optical axis direction of the reference plate in a cantilever fashion at a plurality of positions spaced from each other by a predetermined interval along the circumferential direction of the base, and a bonding part formed in each of the support arms so as to be bonded to the outer peripheral face.
The bonding part may be constituted by an adhesive injection hole penetrating through the support arms from the outer side face to inner side face thereof, whereas a groove extending in the circumferential direction of the base may be formed at a position where the adhesive injection hole is formed in the inner side face.